“When you watch your tiny baby undergoing multiple blood tests, your heart aches. When they bend him back into the foetal position to remove fluid from the base of his spine with a long, threatening-looking needle, it almost breaks,” he says.

It was later discovered Ivan had cerebral palsy and a severe form of epilepsy that led him to have 20 or 30 seizures in a day.

Mr Cameron also pays tribute in his memoirs to “the extraordinary compassion in our health service” and “the best of the NHS” who helped look after his son.

Reflecting on his experience helping care for Ivan, the former Tory leader says: “A world in which things had always gone right for me suddenly gave me an immense shock and challenge.”

“Nothing, absolutely nothing, can prepare you for the reality of losing your darling boy in this way. It was as if the world stopped turning.”

Image copyrightPAImage caption The Cameron family in London in 2006

Mr Cameron’s wife Samantha told the Times 2017 her son’s death “overshadowed everything” and rendered the outside world “meaningless”.

“Like anyone else in my situation, I just kept going. You have to deal with it, because you have no choice.”

She also said it changed her husband’s politics, saying: “It made him understanding, though he couldn’t be too subjective.”